A leather-like sheet such as artificial leather has come to be widely used in clothes, general materials, sport goods, etc. because its superiority to natural leathers such as its light weight and easiness of handling has been accepted by consumers. Known general artificial leathers have been produced by a method roughly including a step of making microfine fiber-forming composite fibers made of two kinds of polymers having different solubilities in solvent into staples; a step of making the staples into a web by using a card, crosslapper, random webber, etc.; a step of entangling the fibers by needle punching, etc. to obtain a nonwoven fabric; a step of impregnating an elastic polymer such as polyurethane in the form of solution in a solvent; and a step of converting the composite fibers to microfine fibers by removing one component of the composite fibers.
However, the staples are relatively easily and unavoidably pulled out or fallen out of the nonwoven fabric body because of their short fiber length. With such an unfavorable tendency, the important surface properties such as a fastness to surface abrasion of napped artificial leathers and an adhesion strength resistant to peeling of grain-finished artificial leathers become insufficient. In addition, an excessive elongation of products and a pull-out of surface fibers occur during the production process, to impair the dense feeling and surface appearance and deteriorate the quality stability.
To solve the above problem, generally employed are, for example, a method of increasing the degree of entanglement of the nonwoven fabric body and a method of increasing the amount of the elastic polymer to be impregnated so as to bind the fibers and strongly constrain the fibers. However, if the degree of entanglement and the amount of the elastic polymer are increased to a level sufficient for solving the above problem, the hand of artificial leather is remarkably impaired. Thus, an artificial leather which satisfies the appearance, hand and surface properties at the same time has not been hitherto realized.
Unlike the production of a short-fiber nonwoven fabric, the production of a long-fiber nonwoven fabric is simple because a series of large apparatuses such as a raw fiber feeder, an apparatus for opening fibers and a carding machine is not needed. In addition, the long-fiber nonwoven fabric is superior to the short-fiber nonwoven fabric in the strength and shape stability. Therefore, the long-fiber nonwoven fabric has been used as the substrate for leather-like sheets. However, only a grain-finished artificial leather having a substrate which is made of long fibers having a normal fineness of 0.5 dtex or more has been on the market. Artificial leathers made of microfine long fibers have not yet been put on the market. This is because that an entangled web having a stable mass per unit is difficult to produce from long fibers, the uneven fineness and strain of composite long fibers likely cause a product-to-product variation, and the dense feeling is poor and the hand likely becomes cloth-like because long fibers are poor in bulkiness as compared with crimped short fibers.
Patent Document 1 proposes a method of preventing the unevenness and improving the bulkiness, in which the nonwoven fabric is densified by partly cutting long fibers to partly relive the strain. However, the proposed method reduces the advantages of long fibers such as improvement in the tenacity and interlaminar peeling strength, and may fail to effectively use the surface abrasion, shape stability, etc. which are characteristic of long fibers. Patent Document 2 proposes to prevent the change of shape of a composite sheet by reinforcing an entangled body of long fibers with a knitted or woven fabric, etc. However, the defect such as wrinkling due to the strain relaxation of fibers cannot be prevented by the mere reinforcement with a fabric. Thus, the appearance and hand and the surface properties are not satisfied at the same time also in artificial leather using a long-fiber nonwoven fabric.
In view of the mechanical strength, fastness to dyeing, hand and appearance of napped surface of leather-like sheet, the elastic polymer is provided into a nonwoven fabric for constituting a fibrous substrate generally by impregnating a solution of a polyurethane elastomer in an organic solvent such as dimethylformamide and then coagulating the solution. However, since a known nonwoven fabric is not sufficient in its shape retention and easily causes pull-out of fibers, a large amount of elastic polymer is needed. Therefore, in a leather-like sheet having napped fibers on its surface, the color unevenness becomes striking because of the difference in the dyeability between a large amount of the impregnated elastic polymer and the fibers, thereby reducing the quality appearance and quality stability. Another problem is that the elastic polymer having exhausted dye thereon falls off during use to remarkably deteriorate the color fastness. In addition, since the rubbery feeling which is characteristic of polyurethane is enhanced, an artificial leather having a natural leather-like dense feeling and flexibility cannot be obtained. In an alternative method, the nonwoven fabric is dyed by a jet dyeing without providing the elastic polymer. However, in the jet dyeing, the nonwoven fabric is repeatedly subject to violent flexing in a high-temperature hot water. Therefore, the nonwoven fabric is largely elongated and torn, and the pull-out of surface fibers is increased, to significantly deteriorate the process passing property (property to be successfully subjected to the treatment intended in each process without problems) and the quality of products being produced. Therefore, this method is hardly applied to industrial production.
To avoid the use of organic solvent in view of protecting the environment and assuring the safety, various methods of producing a leather-like sheet have been proposed, in which an aqueous dispersion of urethane elastomer is used in place of an organic solution of urethane elastomer (Patent Documents 3 and 4). However, as compared with the organic solvent-soluble urethane elastomer, the water-dispersible polyurethane provides a hard hand and is inferior in the napping property of surface fibers and the mechanical strength. In addition, a leather-like sheet impregnated with the water-dispersible polyurethane is extremely poor in the wet color fastness because of the high water absorption and easiness of exhausting dyes of the water-dispersible polyurethane, thereby making its use difficult. In some cases, in addition to the urethane elastomer, an acrylic elastomer may be used to control the hand of knitted or woven fabrics. However, in view of mechanical strength, color fastness, hand and appearance of surface napped fibers, the elastic polymer to be provided into the leather-like sheet has been practically limited to urethane elastomer.    Patent Document 1: JP 2000-273769A    Patent Document 2: JP 64-20368A    Patent Document 3: JP 6-316877A    Patent Document 4: JP 9-132876A